Density arrested BALB/c-3T3 cells treated with platelet derived growth factor (PFGF) become "competent" to replicate DNA. They enter S phase following a lag time of 12 hrs when incubated in medium containing plasma. We have shown that when PDGF-treated competent cells are fused to untreated cells, the resulting heterokaryons become competent to replicate DNA. Cytoplasts derived from PDGF-treated cells are also able to render recipient cells competent. When donor cells are treated with the inhibitors of RNA synthesis actinomycin D or 5, 6-dichloro-beta-ribufuranosylbenzimidazole (DRB) during exposure to PDGF, they are unable to transfer competence; this suggests that transfer of competence is due to a "second message" rather than PDGF itself and that the production of this second message required RNA synthesis. Cycloheximide does not prevent competence transfer indicating that this second message can be RNA. Analysis of cytoplasmic proteins of PDGF-treated cells demonstrates the preferential synthesis of several proteins with moleculr weights ranging from 29K to 70K daltons within 40-90 min. Both the rate of synthesis of one of these proteins (pI) and the percentage of cells induced to become competent are related to the concentrations of PDGF. Pituitary FGF has a similar activity, but EGF, insulin or plasma do not. Treatment of cells with actinomycin D or DRB prevents the synthesis of pI in response to PDGF. The data suggest that the synthesis of cytoplasmic protein(s) like pI is necessary for BALB/c-3T3 cells to become competent.